Battery charges in a hurry

Engineers at Massachusetts Institute of Technology (MIT) have created a new type of battery that could recharge in seconds rather than hours. This breakthrough could lead to a new generation of batteries for green technologies like hybrid vehicles and wind turbines, say the researchers.

The best lithium batteries on the market can squeeze a lot of electrical charge into a small volume but they are a bit sluggish at gaining and discharging that energy. This drawback can put people off buying an electric car. “They have a lot of energy, so you can drive at 55 mph for a long time, but the power is low. You can’t accelerate quickly,” said Gerbrand Ceder, one of the researchers at MIT.

Now Ceder and his colleagues have revamped a standard lithium battery by speeding up the flow of electric charge. They achieve this by applying a lithium phosphate coating to the material’s outer shell. This “simple” idea followed their recent theoretical work, which showed lithium ions travelling along the surface to be just as important as electron flow within the battery.

“The ability to charge and discharge batteries in a matter of seconds rather than hours may open up new technological applications and induce lifestyle changes,” said Ceder.

Speedy ions

Inside a battery, electricity flows via electrons and lithium ions moving towards the anode and cathode respectively. Previous attempts to improve the power of batteries have focussed on increasing the mobility of electrons. Physicists just assumed that lithium ions travel relatively slowly because of their larger size.

Then about five years ago, Ceder and his colleagues modelled a lithium iron phosphate battery and made the surprising discovery that lithium ions should actually be moving at a similar speed to the electrons. The difference is that the ions can only travel through “tunnels” accessed from the surface. The process however is very inefficient because many of ions have trouble entering these tunnels.

Having established this new theory the scientists set out to maximize access of ions to these tunnels. They designed a system, which allows lithium ions to zip quickly around the surface of the material - analogous to cars avoiding traffic by zipping around the ring road of a city. This created a more efficient system where significantly more ions enter into the tunnels

The researchers then created a small battery that could be fully charged or discharged in 20 seconds; without the coating it took 6 minutes. Reporting their findings in Nature they say there is nothing to prevent this battery being scaled up to a hybrid car battery that could be charged in 5 minutes. The only limiting factor will be the available power – 180 kW would be needed for a 15 kW h battery.

“We will now try to prove the same principle for a range of Lithium battery materials. Maybe there are some that have an even higher energy density,” Ceder told physicsworld.com.

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Fracturing

A researcher I spoke to said one of the challenges with new battery materials was that after the batteries were cycled(charged and discharged) many(50-100+) times the surfaces often began to fracture making the batteries commercially unviable because they weren't then able to hold as much charge. This article doesn't say anything about how many cycles they think the batteries will last...